Prototype Update

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The concept for our prototype is based around the issue that Dresser-Rand has noticed with their coupling. They use high pressure and a heavy interference fit to attach the coupling to the shaft. This in turn leads to scratch marks on the shaft when they remove the coupling. Our design eliminates the interference fit and pressure needed to mount the coupling. We have a male and female coupling that will be friction welded onto the shafts. They will be the same outer diameter as the shaft in order to allow components such as a turbine to be removed without removing the coupling. The male coupling has four teeth that fit into the female coupling to transfer the torque. A removable locking collar is used to bolt both couplings together in order to compensate for any axial forces which would pull the coupling apart.

The prototype will be scaled in order to meet the torque and speed requirements provided by Dresser-Rand. The full scale coupling must be able to operate at 9000 rpm and transmit approximately 155,000 in.lbs of torque. We do not have access to machinery which can handle those loads so we must scale down our model and the requirements to test our prototype. If our prototype can meet these goals, then our model will be considered a success.

2 We originally ordered steel in order to match the design requirements from Dresser-Rand. We were ready to machine the components but the machine shop operators told us that it would not be possible to machine the steel that we bought because it would ruin their tools. This led us to build our prototype out of wood which was much easier and quicker to machine. The only problem is that we cannot test the wooden prototype for torque output because it will crack or split. Many of the dimensions needed to be oversized in order to keep the wood from splitting while it was being machined. All components of the prototype have been finished and the model is fully assembled with all of the standard components.
When we initially started to brainstorm ideas for our original power coupling design we decided to use three teeth to transmit the torque. We ran the model in ANSYS and the teeth showed to have very low stress across the face. The teeth were very thick with almost no stress experienced in the opposite contact face. This led us to redesign the teeth by adding a fourth tooth which would make each tooth thinner. The increase in number of teeth also increases the face contact which will transmit the torque. This will help to relieve high stress concentrations on the edges.

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The revised finite element analysis, modeled in ANSYS can be seen in the figures on the left. The coupling was modeled as an assembly with the face of each tooth in full engagement with the female coupling. The figures show only one half of the coupling with the other one hidden in order to see the stress field. The average stress on the face of the teeth and throughout the coupling is below the yield stress for stainless steel. This means that the part will not yield. The torque that we applied is equal to the normal operating torque that the part will experience. When we are actually testing our prototype we will not have access to machinery which will allow us to impart that amount of torque on our coupling. We will need to scale down the model and the torque. This is a good first model which tells us that we are pursuing a design which will function properly. More finite element analysisí will be run in the upcoming week to try and provide more accurate results as well as modeling the locking collar on the shaft. With the full CAD model in ANSYS, we should be able to predict if the part will have an infinite life or if it will run for a certain number of cycles before it needs to be replaced.

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A similar FEA has been underway with the Dresser-Rand coupling geometry. We have been trying to use ANSYS but have been having issues. The main issue was that in order to run the model with the appropriate sized mesh, the model ran for longer than the license that we had access to allowed. We finally worked through the issue and are in the process of modelling the coupling and shaft in ANSYS. We will look at variations in chamfer length, scallop size, and nominal diameter bore size and how they affect the pressure to remove the coupling from the shaft. We will have the models completed by the time that the final report is due.

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